1. Field of the Invention
This invention relates to resin masterbatches having high levels of resin (e.g., at least about 65 wt % of resin), and methods of use of the masterbatches in making polymer articles, such as polyolefin articles. In addition to resin, the masterbatches contain carrier polymer which may be, for example, polyethylene. The carrier polymer is selected based on the processability of the masterbatch and the effect on the final product. In particular, the carrier polymer is preferably selected to facilitate mixing with the resin and/or to facilitate solidification of the molten masterbatch. The masterbatch may be blended with a blend polymer, such as a blend polyolefin, to form a polymer blend, such as a polyolefin blend. The blend may be converted directly to the finished product, e.g., polypropylene film, molded goods, or adhesive, by mixing the masterbatch with the blend polymer, such as polyolefin, and processing the polymer blend into the finished product.
2. Discussion of Background
Polymers are useful in a wide variety of products. For instance, polymers may be used to make films, molded articles, and adhesives.
For instance, polyolefins are plastic materials useful for making a wide variety of valued products due to their combination of stiffness, ductility, barrier properties, temperature resistance, optical properties, availability, and low cost. Since the preferred polyolefins are semi-crystalline polymers, a number of these important properties such as stiffness, barrier properties, temperature resistance, and optical properties, depend on the ability of the polyolefin to crystallize in the most effective manner, and to the desired degree.
The process for forming a polyolefin product strongly affects the crystallization behavior of the material and its ultimate properties. For instance, when polypropylene or polyethylene is cast into thin film, the polymer cools so quickly that the ultimate level of crystallinity is reduced by this "quenching" process, and correspondingly the stiffness of the film is reduced. Cast polypropylene films typically exhibit a stiffness, measured as tensile modulus, of nominally 100 kpsi. Highly oriented polypropylene (OPP) films typically exhibit modulus values 2-4 times higher than the values for cast polypropylene film while non-oriented thick molded articles typically exhibit modulus values nominally 50 to 100% higher than cast polypropylene film. Also when making cast film, it is important that the polypropylene melt solidify quickly to promote high production rates, and also that the crystalline regions which are formed are not so large in size that they confer haze to the film.
Other molded polyolefin articles, particularly thin gauge products made by thermoforming, injection molding, or blow molding, are subject to similar constraints. Faster crystallization which permits rapid demolding and stiffer products is desired, as well as good optical properties promoted by small crystalline domain size.
As a means for improving the stiffness of polyolefins, the addition of a high softening point resin to polyolefins, such as polypropylene and polyethylene, is known. The composition of the resin is preferably such that it exhibits a significantly higher glass transition temperature (T.sub.g) than the amorphous regions of the polypropylene (T.sub.g around -10.degree. C.), and the resin is preferably highly compatible in the polypropylene. It is believed that the effect of the resin is to increase the T.sub.g of the amorphous polypropylene fraction and by doing so increase its tensile modulus at temperatures below 38.degree. C.
The resins described above are friable solids which exhibit very low melt viscosity at the temperatures normally used to process polyolefin. An effective way to blend resin into polyolefin is in a separate compounding step prior to the final use of the blend. It is difficult to incorporate resin into polypropylene or polyethylene during an actual conversion step (for example film casting, sheet extrusion, etc.) because of the dusting characteristics and low melt viscosity of resins.
Accordingly, the use of masterbatches including resin are known to be a preferred way to incorporate resin into polypropylene formulations. U.S. Pat. No. 5,213,744 to BOSSAERT, the disclosure of which is herein incorporated by reference in its entirety, describes a process of forming a masterbatch involving a binary mixture of polyolefin and from 10 to 90 wt %, preferably 20 to 60 wt % of resin. All of the examples of BOSSAERT and claims in BOSSAERT are directed to a binary component masterbatch of polypropylene and resin.
By adding resin to polyolefin in the form of a masterbatch, the resin can be incorporated directly during processing and fabrication of the final product. Thus, the use of a masterbatch eliminates the need for a separate compounding step to incorporate the resin into the polyolefin formulation. Because of economic considerations, it is desirable to achieve as high a resin content in the masterbatch as possible without compromising the ability of the masterbatch to be uniformly blended into the polyolefin, e.g., during extrusion processing.
However, although BOSSAERT discloses the use of masterbatches having high levels of resin, BOSSAERT fails to disclose how to effectively process such high resin content masterbatches into pellets. In this regard, there are difficulties in mixing and pelletizing binary masterbatches of resin and polypropylene.
Regarding the mixing difficulties, the high melting point of polypropylene homopolymer which is about 165.degree. C. for most polypropylene homopolymers hinders effective homogenization of the resin with the polypropylene homopolymer, even when using high shear mixing. With some polypropylenes, the extrudate can be very non-homogeneous, or if processed to a homogeneous state, the peak melting temperature of the masterbatch may be too high such that the melt strength is too low to process easily. For example, the mixing energy required to homogenize the masterbatch may increase the temperature so much that it is difficult to make pellets. As a result of these mixing difficulties, the energy required to compound and homogenize a blend can be high.
Regarding the pelletizing difficulties, the resulting extrudate may not be effectively cut into pellet-like particles by the cutter. For instance, when binary component masterbatches of polypropylene homopolymer and about 50 to 70 wt % resin are made, it is difficult to form pellets because the crystallization is slow such that the material leaving the extruder is too soft. Because the extrudate is soft, the product leaving the pelletizer may be a string of battered polymer, instead of pellets. Under these conditions, the product may also be large lumps and prills, some of which are large enough to put an excessive load on the cutter and dryer motors of the pelletizer. When binary component masterbatches of polypropylene homopolymer and greater than about 70 wt % resin are made, it may be difficult to form pellets because the material leaving the extruder is too brittle. Because the solidified extrudate is brittle, the final product is granulated material, instead of pellets. Thus, the rheology of high resin content masterbatches of polypropylene homopolymer and resin, such as those disclosed by BOSSAERT, prevents the efficient formation of pellets. As a result, in binary resin/polypropylene homopolymer masterbatches, a resin content of about 50 wt % is the maximum amount that can be achieved without severely affecting processing of the masterbatch.
Similar to masterbatches involving polypropylene homopolymer, it is difficult to process most binary masterbatches of propylene copolymer and high levels of resin. In particular, binary masterbatches of propylene copolymer and high levels of resin are often either too soft or too brittle for processing. When the masterbatch is too soft, the masterbatch composition cannot be cut easily into separate pellets. When the masterbatch is too brittle, the final product is granulated material.
As discussed above, adding high softening point resin to polyolefin, such as polypropylene, will increase the glass transition temperature (T.sub.g) of the amorphous phase of the polyolefin and modify its properties. One effect of resin addition is greater stiffness. To achieve significant property modification the resin is preferably added at levels at or above about 2 wt % of the total polyolefin blend. For instance, oriented polypropylene films preferably have about 2 to 30 wt %, more preferably about 5 to 15 wt %, of resin. Cast polypropylene films preferably have about 2 to 10 wt %, more preferably about 3 to 7 wt %, of resin. Polyethylene films preferably have about 2 to 30 wt %, more preferably about 5 to 15 wt %, of resin.
In addition to polyolefins, resin masterbatches may be added to other polymers. For instance, resin masterbatches may be added to adhesives. In adhesive applications, resin masterbatches give fewer handling problems and increase the melt viscosity of the resin component, making the masterbatch easier to blend with the blend polymer of the adhesive.
Adding high levels of resin via a masterbatch having resin and carrier polymer, however, requires that high levels of masterbatch be added which means that a significant amount of carrier polymer is added which often has a negative impact. For instance, in film applications, the carrier polymer may negatively impact haze, ductility and impact properties, formulation cost, and crystallization rate of polymer blends. In adhesive applications, the carrier polymer may negatively affect tack and may increase modulus. It would therefore be desirable to achieve the favorable effects of resin addition while adding lower amounts of carrier polymer, by achieving high resin concentration in masterbatches which are easy to process.